1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly to a cholesteric liquid crystal (CLC) display device.
2. Discussion of the Related Art
Generally, liquid crystal display devices can be classified as either a transmissive liquid crystal display device or a reflective liquid crystal display device. In the transmissive liquid crystal display device, either black or white is displayed by irradiating light from a light source disposed behind a liquid crystal panel to a liquid crystal layer and thus absorbing or transmitting the light depending on an alignment of liquid crystal molecules. Whereas the transmissive liquid crystal display devices require high power consumption because they use an artificial light source behind the liquid crystal panel, the reflective liquid crystal display devices depend on ambient light or an external artificial light source for their light source. Accordingly, the reflective liquid crystal display devices require lower power consumption as compared to the transmissive liquid crystal display devices. Therefore, the need for reflective liquid crystal display devices has been acknowledged.
In reflective liquid crystal display devices, incident light first passes through the liquid crystal panel, then is reflected in the liquid crystal panel, and finally the reflected light passes through the color filter to display a color image. As a result, the utilization ratio of the light is poor in these devices. In addition, the devices require a color filter that has sub-color filters arranged in a regularly repeated order of red (R), green (G), and blue (B). The sub-color-filters red (R), green (G), and blue (B) can be made through processes such as a pigment coating process, a light exposing process, and a patterning process. The manufacturing process for making these color filters is complex and therefore the costs of the color filters is high.
Therefore, a cholesteric liquid crystal (CLC) color filter that uses a cholesteric liquid crystal (CLC) as a color filter has been suggested to overcome the deficiencies described above. The cholesteric liquid crystal (CLC) has a layered structure. The liquid crystal molecules in every layer have similar properties to that of nematic liquid crystals. The alignment of the liquid crystal molecules of each layer can rotate in clockwise or counter clockwise directions enabling a difference in reflectance between the layers. Accordingly, a color can be displayed by the reflection and interference of light that is caused by the difference of the reflectance between layers. The rotations of the cholesteric liquid crystal (CLC) molecules form a helical structure.
A cholesteric liquid crystal (CLC) having a right-handed helical structure reflects a right circular polarization component and transmits a left circular polarization component of the incident light. The incident light consists of right circular polarization and the left circular polarization. If a cholesteric liquid crystal (CLC) has a left-handed helical structure, the cholesteric liquid crystal (CLC) reflects the left circular polarization component and transmits the right circular polarization component of the incident light.
The pitch is an important characteristic in the helical structure of the cholesteric liquid crystal (CLC). The pitch can be understood as a distance between the first cholesteric liquid crystal (CLC) layer and the last cholesteric liquid crystal (CLC) layer when the cholesteric liquid crystal (CLC) molecules in the first cholesteric liquid crystal (CLC) layer rotate 360 degrees. The pitch is a parameter that controls a hue of the cholesteric liquid crystal (CLC). For example, if the pitch is the same with a wavelength of red color (650 nm) then the cholesteric liquid crystal (CLC) reflects the red color observed in a front direction. The pitch of the helical structure of the cholesteric liquid crystal (CLC) can be controlled to selectively reflect or transmit the incident light with a wavelength in a particular range.
The cholesteric liquid crystal (CLC) color filter using the cholesteric liquid crystal (CLC) may be formed by mixing the cholesteric liquid crystal (CLC) with a photo-alignment polymer. Then the mixed solution of the cholesteric liquid crystal (CLC) and the photo-alignment polymer is heated to control the pitch and thereby the cholesteric liquid crystal (CLC) may reflect only the light in a particular wavelength range. The mixed solution of the cholesteric liquid crystal (CLC) is subsequently exposed to ultraviolet light to fix the photo related characteristics of the cholesteric liquid crystal (CLC).
FIG. 1 is a cross-sectional view of a conventional cholesteric liquid crystal (CLC) color filter substrate. As shown in this figure an absorption layer 34 is formed on a substrate 32 to manufacture a cholesteric liquid crystal (CLC) color filter substrate 30. An alignment layer 36 is then formed on the absorption layer 34. A cholesteric liquid crystal (CLC) color filter 38 is subsequently formed on the alignment layer 36. The cholesteric liquid crystal (CLC) color filter 38 includes sub-color filters red (R) 38a, green (G) 38b, and blue (B) 38c. The sub-color filters 38a, 38b and 38c are formed by controlling the pitch of CLC helix to reflect an incident light having a wavelength in a range corresponding to the desired displaying color of each of the sub-color filters red (R) 38a, green (G) 38b, and blue (B) 38c. 
FIGS. 2A and 2B are cross-sectional views illustrating a fabrication process of the cholesteric liquid crystal (CLC) color filter according to the related art. As shown in these figures an absorption layer 34 is formed on a substrate 32 by forming a black resin. An alignment layer 36 is then formed on the absorption layer 34. A cholesteric liquid crystal (CLC) layer 37 is subsequently formed on the alignment layer 36. The cholesteric liquid crystal (CLC) layer 37 is a mixture of the cholesteric liquid crystal (CLC) and a photo-alignment polymer. A plurality of regions are defined in the cholesteric liquid crystal (CLC) layer 37 and each of these regions will become a sub-color-filters red (R), green (G), and blue (B) by controlling the pitch of CLC helix to reflect the incident light having a wavelength in a range corresponding to a desired displaying color of each of the sub-color-filters R 38a, G 38b, and B 38c. 
The pitch of the CLC helix can be controlled in two ways. First, the pitch of the CLC helix can be controlled by varying the temperature, and then exposing the region for sub-color filter R in the cholesteric liquid crystal (CLC) layer 37 to ultraviolet rays using a mask 40 to fix the controlled pitch. Second, the pitch of the CLC helix can be controlled by irradiating ultraviolet rays that have a first wavelength range and then fixing the controlled pitch by irradiating ultraviolet rays that have a second wavelength range. The sub-color filter R for red color can be formed using either of the methods described above. In addition, other sub-color filters, for example, green color and blue color, can be formed through the same process as that of the sub-color filter R. The cholesteric liquid crystal (CLC) color filter 38 can be manufactured by repeating the above-described process for each sub-color-filter R, G, and B. Finally, a passivation layer 42 is formed on the cholesteric liquid crystal (CLC) color filter 38.
If the helical structure of the sub-color-filter R is left-handed, the left circular polarization component of the incident light that is in the wavelength range of the red color is reflected, and other components of the incident light transmits the cholesteric liquid crystal (CLC) color filter and then are absorbed to the absorption layer. Accordingly, an observer can see the red color.